Magneto-mechanical resonators are well known and have been used in retail security applications for decades. In addition, magneto-mechanical resonators (MMRs) are also suitable for buried infrastructure due to their low cost, low profile and flexible components. They can be configured as stand-alone markers or physically attached to an underground pipe or utility. They can be used to identify a buried asset and its location accurately. For example, see US 2012/068823; US 2012/0325359; and US 2013/0099790, each of which is incorporated herein by reference in its entirety.
However, the detection range and frequency stability of such MMRs can be limited.